Which line was selected as the prime meridian at the 1884 International Meridian Conference?

The Greenwich Meridian was selected as the prime meridian at the 1884 International Meridian Conference.

The conference met in Washington, D.C., in October 1884 to establish a common reference for longitude and world timekeeping. Delegates chose the meridian passing through the centre of the transit instrument at the Royal Observatory in Greenwich, England. It became the conventional zero of longitude.

The decision reflected the widespread use of British nautical charts and the Greenwich meridian in international navigation. The conference also adopted a universal day beginning at midnight and divided Earth into 24 time zones in principle, although national time systems developed unevenly afterward.

Modern geodesy uses the International Reference Meridian, which differs slightly from the historical Greenwich line because Earth-centred satellite measurements define coordinates more precisely. Nevertheless, Greenwich remains the familiar name for longitude 0 degrees and is central to the history of Greenwich Mean Time and global navigation.